Conventionally, a substrate-scribing and breaking method has been used for substrates made of glass and other brittle materials in which a mechanical processing means, such as cutter wheel, is used to form a scribe line on the surface of a substrate and then break the scribed substrate by bending the same in such a way that this scribe line facilitates the development of a crack.
Recently, however, in substitution for such a mechanical scribing and breaking method, a substrate scribing and breaking method has been practically applied by irradiating a laser beam to a substrate and thereby forming a vertical crack in the substrate (as disclosed in Patent Document 1).
FIG. 8 is a schematic view depicting the operation of a conventional crack-forming apparatus. According to a crack-forming method using a laser beam as shown in FIG. 8, while a spot beam is irradiated from a laser irradiation unit 102 to a substrate 101 to form an oval beam spot B (laser beam irradiated area), a jet of refrigerant is emitted from a refrigerant nozzle 103 to form a cooling spot (refrigerant emitted area) C.
The substrate 101 is moved in such a direction as being cut out (as shown by arrows in FIG. 8) and the beam spot B is moved relatively to the substrate 101. At this time, the major axis direction of the beam spot B is so arranged as to coincide with the moving direction of the substrate 101. On the other hand, the cooling spot C is so arranged as to be on the continuation of the major axis direction of the beam spot B and at the same time emit a jet of refrigerant to a position behind the beam spot B.
When the oval beam spot B moves in the major axis direction thereof, the area through which the beam spot B passes is continuously heated as long as the beam spot B is passing therethrough at a temperature under the melting point of the substrate, and compressive stress is generated within and around the heated area.
Immediately after the beam spot B heats the area, the cooling spot C passes through the area. As a result, a cooled area is generated adjacent the heated area where the compressive stress was generated and tensile stress is generated around the cooled area. Then, based on difference between the compressive stress and the tensile stress, a vertical crack is vertically formed in the surface of the substrate along the passage of the beam spot B and cooling spot.
Patent Document 1: Japanese Unexamined Patent Publication No. 2001-130921